The present invention relates to a method and an apparatus for the calibration of segmental volume changes through arteries and veins during detection of atherosclerosis.
U.S. Pat. Nos. 5,241,963 to Shankar and 5,297,556 to Shankar disclose a method of detecting the onset and relative degree of atherosclerosis in humans and a method of detecting atherosclerosis while excluding motion artifacts. The Shankar disclosures discuss a volume plethysmograph which, in one embodiment, obtains a waveform which represents arterial blood volume change or segmental volume changes through an artery and compares that volume change signals measured at a plurality of discrete cuff pressure levels. The Shankar volume plethysmograph included a valve and a pump which was used to establish the discrete levels of pressure in a pressure cuff. A pressure transducer generated pressure signals based upon the change in pressure developed by pulsatile blood flow through the artery in the limb. This pulsatile blood flow is called segmental blood volume change herein. To calibrate the volume plethysmograph, the Shankar system first inflated the pressure cuff to a preset level and then injected a small amount of air into the pressure cuff by forcing air from a milled 1 ml chamber. The cuff was then fully deflated and the unit was calibrated electronically based upon the differential pressure signal (resulting in a differential blood volume signal) caused by the injection of the 1 ml volume of air injected to the closed pneumatic system. After the initial calibration and subsequent deflation, the pump was again activated to elevate the pressure in the cuff to a supra diastolic pressure (e.g., 70 mmHg) level. At this level, 1 ml of air was again injected into the cuff via an air chamber and the pressure sensor developed a signal that correlated to the 1 ml injection of air to that particular cuff pressure level. After this initial calibration at 70 mmHg, the volume plethysmograph then began inflating the cuff to pre-determined, discrete cuff pressure levels and measured the pressure change substantially continuously based upon the segmental blood volume change through the artery in the limb under study. The Shankar disclosures indicate that the automatic inflation and deflation, step deflation and calibration are performed by a peristaltic pump and a digital servo loop controlled microprocessor. The Shankar disclosure states "the 1 ml injection of air pressure by chamber 42 and electronic valve 40 into the cuff at any particular time can be utilized to further calibrate the signals." However, the Shankar disclosures also state that the 1 ml chamber cannot be precisely tooled or milled and the electromechanical coupling to the pneumatic system may cause a change from the 1 ml volume standard. Shankar states that "this tooling error and system error is accounted for by an initial calibration with a precise 1 ml reference. This initial calibration will lead to a multiplicative constant A that will change K to A delta P.sub.c." The Shankar disclosures do not show, teach or suggest calibrating the measurement of segmental blood volume change through arteries at each discrete blood pressures as described in the present invention. This calibration at each discrete pressure level is necessary because the gain in the closed pneumatic system changes at each cuff pressure level.
In addition to a volume plethysmograph, the present invention can be utilized in conjunction with an oscillometric method of blood pressure measurement and in conjunction with pulse volume recording (PVR) devices.
U.S. Pat. No. 4,432,374 to Osanai discloses a volume plethysmograph. U.S. Pat. No. 4,144,878 to Wheeler discloses another impedance plethysmograph.